Automobile emissions, as well as the emissions from buses, trucks and other mobile sources utilizing internal combustion engines, continue to be an extremely serious problem, not just in this country but worldwide. Exhaust emissions contain not just solid pollutants, i.e., carbon particulates, but also toxic gases such as carbon monoxide, irritants such as hydrocarbons, oxides of nixtrogen (No.sub.x), and carbon dioxide. While carbon dioxide is not harmful in itself, the carbon dioxide output from growing worldwide combustion of fossil fuels has resulted in an alarming increase in the level of carbon dioxide in the atmosphere, with a concurrent drop in the level of oxygen. The rise in CO.sub.2 level has manifested itself in concern over creating a potentially catastrophic "greenhouse effect".
Mobile sources, specifically automobiles, are considered to be the source of over 75% of the visible air pollution in the United States today. Principally in response to the ongoing harmful effects of uncontrolled fossil fuel emissions, both to humans and the environment, the United States began in the 1970's to force states to impose limits for exhaust emissions in its cars. Most states today require periodic emissions testing of automobiles registered within the state.
In order to meet these emissions standards, most passenger vehicles sold in the U.S., starting from the 1975 model year, have been equipped with catalytic converters. These devices contain a catalytic agent, e.g. one of the noble metals such as platinum, palladium, or rhodium.
Catalytic converters utilize the heat of the exhaust which, promoted by the catalyst, induces chemical reactions in the gases to convert some of the hydrocarbons and harmful pollutants i.e. carbon monoxide, into other compounds such as carbon dioxide and water. The catalysts are present within the housing in one of two forms, monolith (honeycomb structure) or pellets. There are basically two types of catalytic converters, oxidation and three-way. Oxidation converters, which rely on platinum or palladium as the catalyst agent, remove only CO and HC, leaving No.sub.x unchanged. Three-way converters, which act on CO, HC, and NO.sub.x, rely upon platinum, palladium, or rhodium as a catalyst, and in addition require that the engine air-fuel ratio be maintained at, or very near, stoichiometry under all vehicle operating conditions.
At highway speeds, the better catalytic converters are relatively efficient, operating in the range of about 90% efficiency. However, the devices are far from a satisfactory solution to the emissions problem.
As noted before, catalytic converters require heat in order to operate. When the engine is cold or is idling, the efficiency of the converter drops substantially. In fact, estimates show that a major percentage of the emissions in the country occur during idle and cold start, where the catalytic converters are not very effective.
Another serious drawback of catalytic converters is that they require the use of unleaded gasoline. As is well known, unleaded gasoline is more expensive to produce, has lower octane ratings than its leaded counterpart, and gives lower gas mileage. While U.S. consumers have somewhat reluctantly resigned themselves to the use of catalytic converters and unleaded fuels, with the concomitant poor performance and gas mileage that accompanies such use, other countries, for the most part, have not.
Another drawback of catalytic converters is the required use of noble metals such as platinum, palladium, or rhodium. Such metals are rare and thereby expensive. Moreover, the supply of such metals is limited to a few countries, and demand for such metals is growing, and thus there may not be a reliable source for such metals in the future. Further, maintaining effective catalytic function over long period of vehicle operation can be a problem. Catalytic activity will deteriorate due to poisoning of active sites by contaminants, such as lead and phosphorous, and due to exposure to excessively high temperatures. Thus, not only must the vehicle be operated strictly with unleaded fuels, but proper engine maintenance must be followed.
A 1987 published study by the EPA showed that 48% of the cars and light duty trucks in this country are spewing an "unacceptably high" amount of pollution. The reason: malfunctioning emissions equipment.
It is clear that the catalytic converter will not in itself provide a sufficient long term control of air pollution. The EPA has set tougher emissions requirements that will not be met by current catalytic converter systems. What few steps are being taken, e.g. the use of smog pumps or EGR valves, will only make the smog control system more complex and expensive, with little in the way of any real improvement.
Recently, Colorado mandated the use of oxygenated fuels, either corn based ethanol or ether-based MTBE for part of the year to try to reduce the carbon monoxide level on the east slope of the Rocky Mountains. The blended fuels cost about three cents more per gallon to make, and some believe that such fuels, at least if used with a higher content of oxygenated additives, will have negative effects on performance and may cause mechanical problems in the engine.
There have been a great many other proposals for reducing the level of pollutants in exhaust gases, based on the idea of using electrical fields to ionize particles. These proposals fall into several categories.
One approach has been to establish a corona discharge in the flow path of the gases, i.e., to bombard the gases with electrons. An example of this type of device is disclosed in U.S. Pat. No. 4,380,900 assigned to Robert Bosch GmbH. As do most of these patents, the Bosch device attempts to remove particulate matter (soot) only. One patent, Sikich U.S. Pat. No. 3,979,193, sets up an alternating current corona discharge to set up an ozone field that, according to the inventor, acts to convert carbon monoxide into carbon dioxide. He does not indicate the degree to which this occurs, but if in fact ozone is created and emitted such would be extremely undesirable.
Other types of devices employ electrostatic or alternating current fields without corona discharge, but once again these devices are intended, for the most part, to ionize and trap particulates. For example, Lui U.S. Pat. Nos. 4,304,096, 4,316,360, 4,338,784, and 4,355,504 propose devices with particle collector plates, charged to several thousand volts dc, to collect electrically charged particles in diesel exhaust gases. In Yang U.S. Pat. No. 4,376,637, carbon particles in diesel exhaust are vaporized by a dc voltage grid and oxidize to form carbon dioxide. Stanton U.S. Pat. No. 4,098,578 discloses a device which adds water, hydrogen peroxide, or alcohol to the exhaust gases and then subjects the mixture to an intermittent DC field to ionize the unburned hydrocarbons and additives. Newbold et al. U.S. Pat. No. 3,620,008 discloses a device that subjects the exhaust stream to a high frequency electrical field of about 10,000 volts AC. A charged graphite scepter is provided in the inlet to set up an ignition plasma reaction, in which the gas stream is heated to about 10,000 .degree. F.
None of these proposals involving the use of electrostatic, alternating current, or corona discharge fields have been adopted by the automobile industry. As noted above, to the extent the industry has adopted any emission treatment device, it has been the catalytic converter.